The present invention relates to digital data communication systems and more particularly to a technique for controlling transmission errors in such systems.
There are basically two known categories of schemes for controlling transmission errors in data communication systems: the automatic-repeat-request (ARQ) scheme and the forward-error-correction (FEC) scheme. The ARQ scheme is widely used because it is simple and it provides high system reliability. However, systems using ARQ scheme have a severe drawback, their throughput falls rapidly with increasing channel error rate. For channels such as satellite channels with large round trip delay, this problem becomes even worse. Systems using the FEC scheme maintain a constant throughput regardless of the channel error rate. However, the FEC scheme is less reliable than the ARQ scheme. Moreover, it is very difficult to implement when a large collection of error patterns is to be corrected.
There are basically three types of ARQ schemes, these being the Stop-and-Wait, Go-Back-N, and Selective Repeat. In any of these three techniques, the receiver sends the sender the results of the error detection in D.sub.j, C(D.sub.j). If no error is detected, a positive acknowledgement (ACK) is sent. Otherwise, a negative acknowledgement (NAK) is sent. The three techniques differ in what the receiver and sender do between the instant the sender has completed transmitting D.sub.j and the instant the sender receives the acknowledgement for D.sub.j. In Stop-and-Wait, nothing is done in that period. In selective reject, with a limited number of outstanding packets allowed, transmission of new packets and/or repeat of the packets for which a NAK has been received may occur in that period. In Go-Back-N, the N-1 packets which follow D.sub.j are transmitted in that period. Besides the difference described above, these techniques may differ in the length of the interval between the instant a NAK is received (if an error is detected in D.sub.j,C(D.sub.j) for D.sub.j and the instant D.sub.j is repeated.
Although the Stop-and-Wait ARQ is simple, it is inherently inefficient due to the idle time spent waiting for an acknowledgement for each transmitted data block. This inefficiency becomes unacceptable for systems where transmission rate is high and round trip propagation delay is large, such as in satellite channels. Selective Repeat ARQ is the most efficient scheme among the three types, but has implementation problems such as infinite buffer requirement or buffer overflow, if a buffer of finite size is used. The Go-Back-N ARQ, while being less efficient than the Selective Repeat ARQ, is more efficient than the Stop-and-Wait ARQ and its implementation is simpler than that of the Selective Repeat ARQ.
In a Go-Back-N ARQ system, the transmitter sends data blocks continuously to the receiver and the receiver sends acknowledgements to the transmitter continuously. When a data block is received successfully, a positive acknowledgement (ACK) is sent to the transmitter. However, when a data block is detected in error, a negative acknowledgement (NAK) is sent to the transmitter. On receiving a NAK, the transmitter backs up to the data block that was detected in error at the receiver and retransmits that data block and subsequent data blocks transmitted in the interval between the original transmission and the receipt of the NAK. At the receiver, the data blocks following the erroneously received data blocks are discarded no matter whether they are received correctly or not. This scheme is simple. However, since it usually involved retransmitting good data blocks following an erroneous data block, its efficiency, the maximum achievable throughput, becomes unacceptably low when channel error rate is high and round trip propagation is large.
In a Go-Back-N system, after the last data block has been transmitted, the transmitter stays idle until arrival of the next data block or a NAK of a previously transmitted data block. In a recently proposed Go-Back-N scheme, during the period of time when the transmitter would be normally idle under the Go-Back-N scheme, it repeatedly transmits the last data block. Although such Go-Back-N scheme improves the response time from the data block entering the transmitter until it being output by the receiver, its efficiency is still the same as the basic Go-Back-N ARQ.
It is an object of the present invention to control transmission errors in data communication systems in a simple and efficient manner.
It is another object to control transmission errors in a more efficient and reliable manner than otherwise achieved with the known automatic repeat request (ARQ) and forward error correction (FEC) schemes.
It is a further object to improve transmission efficiency when a data communication channel is very noisy and the round trip propagation delay is large.